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Halszkaraptorinae Cau et al., 2017 Revised diagnosis. Small dromaeosaurids that possess dorsoventrally flattened premaxillae, premaxillary bodies perforated by many neurovascular foramina, enlarged and closely packed premaxillary teeth that utilized delayed replacement patterns, reduced anterior maxillary teeth, dorsolateral placement of retracted external nares, greatly elongated cervical vertebrae, anterior cervical vertebrae with round lobes formed by the postzygapophyses, horizontal zygapophyses, and pronounced zygapophyseal laminae in the anterior caudal vertebrae, mediolaterally compressed ulnae with sharp posterior margins, second and third metacarpals with similar thicknesses, shelf-like supratrochanteric processes on the ilia, elongated fossae that border posterolateral ridges on the posterodistal surfaces of the femoral shafts, and third metatarsals in which the proximal halves are unconstricted and anteriorly convex., Published as part of Lee, Sungjin, Lee, Yuong-Nam, Currie, Philip J., Sissons, Robin, Park, Jin-Young, Kim, Su-Hwan, Barsbold, Rinchen & Tsogtbaatar, Khishigjav, 2022, A non-avian dinosaur with a streamlined body exhibits potential adaptations for swimming, pp. 1-9 in Communications Biology 5 (1) on page 2, DOI: 10.1038/s42003-022-04119-9, http://zenodo.org/record/7390392, {"references":["4. Cau, A. et al. Synchrotron scanning reveals amphibious ecomorphology in a new clade of bird-like dinosaurs. Nature 552, 395 - 399 (2017)."]}

Natovenator polydontus gen. et sp. nov. Holotype. MPC-D 102/114 (Institute of Paleontology, Mongolian Academy of Sciences, Ulaanbaatar, Mongolia) is a mostly articulated skeleton with a nearly complete skull (See Supplementary Table 1 for measurements). Locality and horizon. Baruungoyot Formation (Upper Cretaceous), Hermiin Tsav, Omnogovi Province, Mongolia 13 (Supplementary Fig. 5). (2022) 5:1185 | https://doi.org/10.1038/s42003-022-04119-9 | www.nature.com/commsbio Etymology. Natovenator, from the Latin nato (swim) and venator (hunter), in reference to the hypothesized swimming behaviour and piscivorous diet of the new taxon; polydontus, from the Greek polys (many) and odous (tooth) in reference to the unusually many teeth. Diagnosis. A small halszkaraptorine dromaeosaurid with the following autapomorphies: wide groove delimited by a pair of ridges on the anterodorsal surface of the premaxilla, premaxilla with an elongated internarial process that overlies nasal and extends posterior to the external naris, 13 premaxillary teeth with large and incisiviform crowns, first three anteriormost maxillary teeth are greatly reduced and are clustered together with the following tooth without any separations by interdental septa, anteroposteriorly long external naris (about 30% of the preorbital skull length), paroccipital process with a anteroposteriorly broad dorsal surface, elongate maxillary process of the palatine that extends anteriorly beyond the middle of the antorbital fenestra, pterygoid with a deep fossa on the medial surface of the quadrate ramus, distinct posterolaterally oriented projection on the lateral surface of atlas, absence of pleurocoels in cervical vertebrae (not confirmed in the missing fifth cervical centrum), posterolaterally oriented and nearly horizontal proximal shafts in the dorsal ribs, hourglass-shaped metacarpal II with distinctly concave medial and lateral surfaces. Description. The skull of Natovenator is nearly complete, although the preorbital region has been affected by compression and is slightly offset from the rest of the skull (Figs. 1c, d, 2a–d and Supplementary Figs. 1, 2). Near the tip of the snout, the premaxilla is marked by a broad groove. The body of the premaxilla is also dorsoventrally low and is perforated by numerous foramina that lead into a complex network of neurovascular chambers (Supplementary Fig. 1b) as in Halszkaraptor 4. Similarly, the external naris is positioned posteriorly and is level with the premaxilla-maxilla contact (Fig. 2a, b), although it is marginally behind this position in Halszkaraptor 4. It is also dorsally placed compared to those of other non-avian theropods and faces dorsolaterally. The exceptionally long external naris and accordingly elongated internarial process of Natovenator (Fig. 2c) are unique among dromaeosaurids but comparable to those in aquatic toothed birds 14 as well as in therizinosaurs 15, 16. The frontal is similar to those of other halszkaraptorines 4, 17 in that it is vaulted to accommodate a large orbit and has little contribution to the supratemporal fossa. A sharp nuchal crest is formed by the parietal and the squamosal (Supplementary Fig. 2a–e). The latter also produces a shelf that extends over the quadrate head as in other dromaeosaurids 18. The paroccipital process curves gently on the occiput and has a broad dorsal surface that tapers laterally (Fig. 2f and Supplementary Fig. 2b, e). Its ventrolateral orientation is reminiscent of Mahakala 17 but is different from the more horizontal paroccipital process of Halszkaraptor 4. The occipital condyle is long and constricted at its base. A shallow dorsal tympanic recess on the lateral wall of the braincase is different from the deep one of Mahakala 17. The palatine is tetraradiate with a greatly elongated maxillary process, which extends anteriorly beyond the level of the mid-antorbital fenestra. The pterygoid is missing its anterior portion (Fig. 2g and Supplementary Fig. 2a–e). A deep fossa on the medial surface of the thin quadrate ramus is not seen in any other dromaeosaurids. The mandibles of Natovenator preserve most of the elements, especially those on the left side (Fig. 1a, b, d and Supplementary Figs. 1a, 2). Each jaw is characterized by a slender dentary with nearly parallel dorsal and ventral margins, a surangular partially fused with the articular, a distinctive surangular shelf, and a fan-shaped retroarticular process that protrudes dorsomedially. The upper dentition of Natovenator is heterodont as the premaxillary teeth are morphologically distinct from the maxillary teeth (Fig. 2a, b, e and Supplementary Fig. 1a, c). There are unusually numerous premaxillary teeth tightly packed without any separation of the alveoli by bony septa. The roots of the teeth are long, and the crowns are tall and incisiviform as in Halszkaraptor 4. Moreover, the large replacement teeth in the premaxilla suggest that the replacement of the premaxillary teeth was delayed as in Halszkaraptor 4. However, the number of teeth in each premaxilla is 13 in Natovenator, whereas it is only 11 in Halszkaraptor 4. In the maxilla, the three most anterior maxillary teeth are markedly shorter than the premaxillary teeth and the more posterior maxillary teeth. This pattern is also observed in Halszkaraptor, although the number of shorter maxillary teeth differs as it has two reduced ones 7. Both the maxillary and dentary teeth have sharp fang-like crowns that lack serrations. Although posteriormost parts are poorly preserved, there are at least 23 alveoli in each of the maxilla and dentary, which suggests high numbers of teeth in both elements. (2022) 5:1185 | https://doi.org/10.1038/s42003-022-04119-9 | www.nature.com/commsbio (2022) 5:1185 | https://doi.org/10.1038/s42003-022-04119-9 | www.nature.com/commsbio The neck of Natovenator, as preserved, is twisted and includes ten elongated cervical vertebrae, although most of the 5th cervical is missing (Figs. 1, 3a–d). This elongation of the cervicals results in a noticeably longer neck than those of most dromaeosaurids and is estimated to be longer than the dorsal series. It is, however, proportionately shorter than that of Halszkaraptor, which has a neck as long as its dorsal and sacral vertebra combined 4. Another peculiarity in the neck of the Natovenator is a pronounced posterolaterally extending projection on the neurapophysis of the atlas (Fig. 3a and Supplementary Fig. 2b, c, e). The postzygapophyses of each anterior cervical are fused into a single lobe-like process as in Halszkaraptor 4. Pleurocoels are absent in the cervical vertebrae. In contrast, Halszkaraptor has pleurocoels on its 7th–9th cervicals 4. A total of 12 dorsal vertebrae are preserved (Figs. 1a, b, 3e, 4a and Supplementary Figs. 3a–d). They all lack pleurocoels, and their parapophyses on the anterior and middorsals are placed high on the anterodorsal end of each centrum. Interestingly, the positions of the parapophyses are similar to those of hesperornithiforms 19 – 21 rather than other dromaeosaurids such as Deinonychus 22 or Velociraptor 23. The preserved dorsal ribs, articulated with the second to seventh dorsals, are flattened and posteriorly oriented (Figs. 1, 3e, 4a–d). The proximal shafts are also nearly horizontal, which is indicative of a dorsoventrally compressed ribcage. Each proximal caudal vertebra has a long centrum and horizontal zygapophyses with expanded laminae (Fig. 3f and Supplementary Fig. 3e–i), all of which are characters shared with other halszkaraptorines 4, 17. The forelimb elements are partially exposed (Figs. 1a, b, 2a–d, 3e, g). The nearly complete right humerus is proportionately short and distally flattened like that of Halszkaraptor 4. The shaft of the ulna is mediolaterally compressed to produce a sharp posterior margin as in Halszkaraptor 4 and Mahakala 17. Metacarpal III is robust and is only slightly longer than metacarpal II. Similarly, metacarpal III is almost as thick and long as other second metacarpals of other halszkaraptorines 4, 17. The femur has a long ridge on its posterior surface, which is another characteristic shared among halszkaraptorines 4. Typically for a dromaeosaurid, metatarsals II and III have ginglymoid distal articular surfaces (Fig. 3h and Supplementary Fig. 4f, h). The ventral surface of metatarsal III is invaded by a ridge near the distal end, unlike other halszkaraptorines (Fig. 3h) 4, 5, 17, 24. (2022) 5:1185 | https://doi.org/10.1038/s42003-022-04119-9 | www.nature.com/commsbio Phylogenetic analysis. The phylogenetic analysis found more than 99,999 most parsimonious trees (CI = 0.23, RI = 0.55) with 6574 steps. Deinonychosaurian monophyly is not supported by the strict consensus tree (Supplementary Fig. 6). Instead, Dromaeosauridae was recovered as a sister clade to a monophyletic clade formed by Troodontidae and Avialae, which is consistent with the results of Cau et al. 4 and Cau 7. Halszkaraptorinae is positioned at the base of Dromaeosauridae as in Cau et al. 4, although there are claims that dromaeosaurid affinities of halszkaraptorines are not well supported 25. Nine (seven ambiguous and two unambiguous) synapomorphies support the inclusion of Halszkaraptorinae in Dromaeosauridae. The two unambiguous synapomorphies are the anterior tympanic recess at the same level as the basipterygoid process and the presence of a ventral flange on the paroccipital process. A total of 20 synapomorphies (including one unambiguous synapomorphy) unite the four halszkaraptorines, including Natovenator (Supplementary Fig. 7). In Halszkaraptorinae, Halszkaraptor is the earliest branching taxon, and the remaining three taxa form an unresolved clade supported by three ambiguous synapomorphies (characters 121/ 1, 569/0, and 1153/1). Two of these synapomorphies are related to the paroccipital process (characters 121 and 569), which is not preserved in Hulsanpes 5, 24. The other is the presence of an expansion on the medial margin of the distal half of metatarsal III, which is not entirely preserved in the Natovenator. When scored as 0 for this character, Natovenator branches off from the unresolved clade. It suggests that the medial expansion of the dorsal surface of metatarsal III could be a derived character among halszkaraptorines.

Zanabazar, new genus TYPE SPECIES: Zanabazar junior (Barsbold, 1974). DERIVATION OF NAME: In honor of Zanabazar (1635–1723), the first Bogd Gegen of Mongolia. INCLUDED SPECIES: Type species only. LOCALITY AND AGE: Nemegt Formation at Bugiin Tsav, Omnogov Aimag, Mongolia (figs. 2, 21). The Nemegt Formation overlies the Djadokhta Formation and is considered to be Maestrichtian based primarily upon the vertebrate fauna (Jerzykiewicz and Russell, 1991). DIAGNOSIS: Differs from Saurornithoides mongoliensis and most other troodontids (save Troodon formosus) in lacking a small depression on the lateral wall of the braincase just dorsal to the trigeminal foramen. The holotype of Zanabazar junior is the largest published troodontid specimen (midline length of the skull 5 272 mm), and is substantially larger than most other troodontid species. Only Troodon formosus appears to approach it in size. Differs from Troodon formosus in having a more ovoid-shaped foramen magnum, a posttemporal fenestra that is enclosed by an almost equal proportion of the exoccipital/opisthotic and squamosal (this same feature in Troodon formosus is formed almost entirely by the exoccipital/opisthotic), a deeper paroccipital process resulting in a different positon of the posttemporal fossa relative to the foramen magnum, and the lack of an osseous signature of the ophthalmic branch of the trigeminal nerve (CN V) in the laterosphenoid.

Zanabazar junior (Barsbold, 1974) HOLOTYPE: IGM 100 /1, a skull and partial mandible, six sacral and 11 caudal vertebrae, and the distal part of the right hind limb. The skull is largely complete except for the left and right quadrate, quadratojugal, ventral part of the lacrimal, palatine, pterygoid, and ectopterygoid, and the left squamosal, postorbital, and jugal, posterior part of the right jugal, and the postdentary bones other than the splenials. The posterior part of the left nasal is reconstructed in plaster, as are small parts of the left maxilla anterior to the maxillary fenestra and dorsal to the interfenestral bar. Plaster covers the ventral surface of the bones roofing the antorbital fenestra. The mandible comprises only the dentaries and splenials. The right hind limb includes the distal end of the tibia with the coossified astragalus and calcaneum, a distal tarsal, and the proximal ends of MT II–IV. TYPE LOCALITY AND AGE: As for Zanabazar. DIAGNOSIS: As for Zanabazar. REFERRED SPECIMENS: No new material of this species has been described, but two other troodontids have been described from the Nemegt Formation of Mongolia. Borogovia gracilicrus Osmólska, 1987, is known only from a partial hind limb from Altan Uul IV, and Tochisaurus nemegtensis Kurzanov and Osmólska, 1991, is known only from a metatarsus from the Nemegt area. Because the postcranial elements of Zanabazar junior do not include those parts diagnostic of the other two species, it is possible that one of the other two Nemegt Formation taxa is a junior synonym. COMMENTS: We chose to construct the name Zanabazar for Saurornithoides junior based on the morphological differences between the holotype and that of Saurornithoides mongoliensis and the questionable monophyly of Saurornithoides as defined by the sistertaxon relationship of S. mongoliensis and S. junior. This decision also was influenced by the common practice in phylogenetic analyses of assuming monophyly by compositely coding taxa (especially ‘‘genera’’). The differences between Zanabazar junior and Saurornithoides mongoliensis certainly merit coding these taxa separately. The temporal difference separating these specimens was not, and should not, be used to support taxonomic decisions., Published as part of Norell, Mark A., Makovicky, Peter J., Bever, Gabe S., Balanoff, Amy M., Clark, James M., Barsbold, Rinchen & Rowe, Timothy, 2009, A Review of the Mongolian Cretaceous Dinosaur Saurornithoides (Troodontidae: Theropoda), pp. 1-64 in American Museum Novitates 3654 on page 28, DOI: 10.1206/648.1, http://zenodo.org/record/5357410, {"references":["Barsbold, R. 1974. Saurornithoididae, a new family of small theropod dinosaurs from central Asia and North America. Palaeontologia Polonica 30: 5 - 22.","Osmolska, H. 1987. Borogovia gracilicrus gen. et sp. n., a new troodontid dinosaur from the Late Cretaceous of Mongolia. Acta Palaeontologica Polonica 31: 133 - 150.","Kurzanov, S. M., and H. Osmolska. 1991. Tochisaurus nemegtensis gen. et sp. n., a new troodontid (Dinosauria, Theropoda) from Mongolia. Acta Palaeontologica Polonica 36: 69 - 76."]}

Norell, Mark A., Makovicky, Peter J., Bever, Gabe S., Balanoff, Amy M., Clark, James M., Barsbold, Rinchen, Rowe, Timothy (2009): A Review of the Mongolian Cretaceous Dinosaur Saurornithoides (Troodontidae: Theropoda). American Museum Novitates 3654: 1-64, DOI: 10.1206/648.1, URL: http://www.bioone.org/doi/abs/10.1206/648.1

Saurornithoides Osborn, 1924 TYPE SPECIES: Saurornithoides mongoliensis Osborn, 1924, by original designation. INCLUDED SPECIES: Only the type species is currently referred to Saurornithoides. LOCALITIES AND AGE: Saurornithoides is known with certainty only from the location of the holotype of Saurornithoides mongoliensis —Djadokhta Formation of Bayan Zag, upper Cretaceous of Mongolia (fig. 2). DIAGNOSIS: Differs from Zanabazar junior in its smaller overall size, lower number of teeth (108 versus 118), the presence of a jugal that is straight beneath the round orbit rather than curved, and the possession of a pneumatic recess on the lateral surface of the prootic dorsal to the trigeminal fenestra (the pneumatic recess is also absent in Troodon formosus). The maxillary teeth of Saurornithoides exhibit some increase in crown height posteriorly but to a lesser degree than in Zanabazar junior. Also in contrast to Zanabazar junior, the maxillary teeth of Saurornithoides lack replacement gaps. In these latter two characters, Saurornithoides agrees with Mei long, Sinovenator changii, and basal avialans among paravians, whereas Zanabazar junior is more similar to Byronosaurus jaffei and dromaeosaurs. COMMENT: The monophyly of Saurornithoides as historically recognized—including Saurornithoides mongoliensis and Saurornithoides junior —has not previously been questioned. A sister-taxon relationship between these two taxa, however, is difficult to support empirically. This may be due in part to the overall poor preservation of the holotype of Saurornithoides mongoliensis. Morphological disparity does exist between the known specimens of these two taxa and, although this list of differences is less than we might normally expect when taxonomically separating specimens at the generic level, the retention of junior in Saurornithoides implies a privileged relationship with mongoliensis that cannot currently be supported. The one polarized character previously considered to support the monophyly of mongoliensis + junior was the position of the facial foramen (CN VII) within the lateral depression of the braincase (Turner et al., 2007b). It now appears likely that this feature also is present in Troodon formosus and therefore is derived at a slightly more inclusive position on the troodontid tree (see below)., Published as part of Norell, Mark A., Makovicky, Peter J., Bever, Gabe S., Balanoff, Amy M., Clark, James M., Barsbold, Rinchen & Rowe, Timothy, 2009, A Review of the Mongolian Cretaceous Dinosaur Saurornithoides (Troodontidae: Theropoda), pp. 1-64 in American Museum Novitates 3654 on page 4, DOI: 10.1206/648.1, http://zenodo.org/record/5357410, {"references":["Osborn, H. F. 1924. Three new Theropoda, Protoceratops zone, central Mongolia. American Museum Novitates 144: 1 - 12.","Turner, A. H., D. Pol, J. A. Clarke, G. M. Erickson, and M. A. Norell. 2007 b. A basal dromaeosaurid and size evolution preceding avian flight. Science 317: 1378 - 1381."]}

Norell, Mark A., Clark, James M., Turner, Alan H., Makovicky, Peter J., Barsbold, Rinchen, Rowe, Timothy (2006): A New Dromaeosaurid Theropod from Ukhaa Tolgod (Ömnögov, Mongolia). American Museum Novitates 3545 (1): 1-51, DOI: 10.1206/0003-0082(2006)3545[1:ANDTFU]2.0.CO;2, URL: http://www.bioone.org/perlserv/?request=get-abstract&doi=10.1206%2F0003-0082(2006)3545%5B1%3AANDTFU%5D2.0.CO%3B2